The objectives of this proposal are to characterize the epidemiology of syphilis among IDUs in Tijuana, with respect to individual, social network and environmental and structural factors, and to apply a combination of epidemiological, geographical and network methods to inform and develop a structural-level and/or peer-based sexual risk reduction intervention. This addresses NIDA's Division of Epidemiology, Services and Prevention Research mission statement to "improve the nation's public health by promoting integrated approaches to understand and address interactions between individuals and environments that contribute to the continuum of problems related to drug use." During the first two mentored years of the project, the applicant will undertake coursework and independent studies to build her knowledge base and skills in the areas of social network analyses, multi-level modeling and geographic information systems (GIS). Practical application of these skills will be obtained through modeling individual, network and geographical aspects of syphilis transmission among IDUs in Tijuana participating in the 'Proyecto El Cuete', a prospective cohort of IDUs established by the primary mentor, Dr. Steffanie Strathdee. During the independent phase of the project, the applicant will apply the information gained through these methods to inform and develop a structural-level sexual risk reduction intervention. The intervention will focus on high risk groups identified during the first phase, and will focus on structural-level factors, applying either venue-based and/or peer-outreach methods, depending on the results from phase one and the level of acceptance and feasibility of these methods within the community. Initial focus groups within the community of interest will be used to assess and develop different ideas for the intervention, and the effectiveness of the intervention will be tested using a pre/post-intervention test design. STIs remain an important burden globally, both due to their own long-term complications, as well as their impact on HIV transmission. In addition to the immediate benefit of increased testing and treatment among the high risk populations in Tijuana and, by extension, the connected communities in San Diego, this proposal sets out to derive new combinations of methods to effectively and efficiently assess transmission patterns among high risk groups and uncover contextual and structural factors for application in the development of interventions. [unreadable] [unreadable] [unreadable]